Driven by growing demand on transmission capacities, sophisticated modulation formats with improved spectral efficiency are becoming the established technology-of-choice for commercial usage in optical communication systems.
Relatively robust and price effective on-off keying, widely applied for data rates of up to 10 Gbit/s, is step by step replaced by transmission formats modulated in phase and polarization, enabling further increase of data rates to 40 Gbit/s, 100 Gbit/s and higher.
Hereto, combination of polarization multiplexing and coherent signal detection, namely “coherently-detected polarization multiplexed quadrature phase shift keying” (CP-QPSK), has been identified as a modulation format of choice for next generation high capacity transmission.
The main CP-QPSK drivers are that it doubles the spectral efficiency and the total capacity, it is part of the 100 G standardization according to the Optical Internetworking Forum (OIF), and all major system houses and component suppliers are currently working on CP-QPSK solutions.
On system level, it is of interest to reuse already implemented concepts when introducing new modulation formats.
This applies also for optical pump sources that are currently used in context with different applications.
As an example of applications for optical pump sources it is worthwhile to mention high power pump lasers for Raman amplifiers, which enable a further increase of span length or idler channels which guarantee optimum EDFA operation in Dense Wavelength Division Multiplexing (DWDM) systems with limited total channel count as well as link stabilization in submarine transmission systems, or dynamically controlled Continuous Wave (CW) channels, which allow for network stabilization in case of sudden power transients (e.g. due to fiber cut).
In all of the aforementioned applications, the power level of the pump signal is usually higher than the average power level of the in-service data signals. In order to prevent polarization dependent gain ripple or signal distortions due to FWM interactions or polarization-dependent Raman gain, an unpolarized CW light or a filtered ASE source is conventionally employed.
All these techniques are well known in the art and used in current Dense Wavelength Division Multiplexing (DWDM) systems with conventional modulation formats as, for example, On Off Keying (OOK), Optical Duo Binary (ODB), Differential Phase Shift Keying (DPSK) or Differential Quadrature Phase Shift Keying (DQPSK).
However, much more than other modulation formats, modulation formats based on polarization multiplexing are sensitive to random or deterministic polarization rotations induced by cross-polarization modulation (XPolM) between the optical signals propagating along a transmission link.
In particular, a critical role is played by the spectral region in which such polarization rotations fall. Accordingly, the following classification can be introduced:                Slow polarization rotations, whose spectral components fall within the adaptive compensation speed of the receiver (<100 kHz). These rotations are typically caused by mechanical and thermal stress effects cumulating during fiber propagation and the receiver is designed to fully compensate for them.        Fast polarization rotations, whose spectral components exceed the adaptive compensation speed of the receiver but fall within its electrical bandwidth (between 100 kHz and the polarization tracking cut-off frequency). These polarization rotations result from XPolM effects cumulating during fiber propagation and the adaptive algorithms within the receiver are not fast enough to compensate for them.        Ultra-fast polarization rotations, whose spectral components fall outside the electrical bandwidth of the receiver (above the polarization tracking cut-off frequency). These polarization rotations also result from XPolM effects cumulating during fiber propagation, but they are so fast that they are rejected by the electrical filter of the receiver and therefore must not be compensated for by the adaptive algorithms of the receiver itself.        
FIG. 1 is a schematic representation of the application of a conventional depolarization technique. In particular, FIG. 1 shows a combination of two delayed fractions of a single CW light 11, the first fraction over a Variable Optical Attenuator 14 and the second fraction over an optical fiber 13 characterized by a fiber length L>Lcoh, where Lcoh is the coherence length.
FIG. 2 is a schematic representation of the application of a conventional depolarization technique. In particular, FIG. 2 shows the combination 22 of two orthogonally polarized CW signals 23 and 26 from a single laser source 21. It shows also the Polarization Beam Splitter PBS 24, which splits the single laser source 21 in two orthogonally polarized CW signals 23 and 26, and a Polarization Beam Combiner PBC 25, which combines the two orthogonally polarized CW signals 23 and 26.
FIG. 3 is a representation 31 of the degradation of the Bit Error Ratio BER 32 of a 40 G CP-QPSK optical signal due to a depolarized CW channel. In particular, FIG. 3 shows the Bit Error Ratio BER 32 of a 40 G CP-QPSK optical signal versus the difference between the power of the Continuous Wave CW pump signal and the power of CP-QPSK optical signal for three different cases:                a combination 34 of two delayed fractions of a single CW light, the CW signal being located 50 GHz away from the CP-QPSK optical signal;        a combination 35 of two orthogonally polarized CW signals and from a single laser source, the CW signal being located 50 GHz away from the CP-QPSK optical signal;        a combination 36 of two orthogonally polarized CW signals and from a single laser source, the CW signal being located 2.6 THz away from the CP-QPSK optical signal.        
In the example, the CW pump and 40 G CP-QPSK co-propagate over a 700 km fiber link. The CW power level has been continuously increased relatively to the data signal. As shown in FIG. 3, significant bit-error ratio degradations in the CP-QPSK signal can be observed starting at power differences of 1 dB. When the CW signal is a direct neighbor of the CP-QPSK data signal FEC threshold of 10−3 is violated when exceeding 5 dB pump-data power difference. Furthermore, significant distortions have been detected even when the CW signal is located 2.5 THz away from the CP-QPSK channel.
The example represented in FIG. 3 clearly shows that a conventional depolarized CW signal causes significant penalties for polarization multiplexed data signals over a very wide spectral region, whose width depends on the power of the CW signal itself. For this reason, due to high power difference, similar distortions can be expected also from co-propagating Raman pumps, thus strongly limiting its application in optical transmission systems with polarization-multiplexed channels.
As an alternative, the use of filtered Amplified Spontaneous Emission (ASE) light has been investigated. In contrary to intentionally depolarized CW signals characterized by fast but deterministic polarization rotations, ASE light is completely depolarized, therefore polarization rotations are completely randomized and all polarization states are represented in it with the same probability.
FIG. 4 is a representation 41 of the degradation of the Bit Error Ratio BER 42 of a 40 G CP-QPSK optical signal due to a Amplified Spontaneous Emission (ASE) source. In particular, FIG. 4 shows the Bit Error Ratio BER 42 of a 40 G CP-QPSK optical signal versus the difference between the power of the Amplified Spontaneous Emission (ASE) source and the power of CP-QPSK optical signal for three different cases:                the Amplified Spontaneous Emission (ASE) source is located 350 GHz away 44 from the CP-QPSK optical signal;        the Amplified Spontaneous Emission (ASE) source is located 850 GHz away 45 from the CP-QPSK optical signal;        the Amplified Spontaneous Emission (ASE) source is located 2.60 THz away 46 from the CP-QPSK optical signal.        
For this experiment, a wide-band, Amplified Spontaneous Emission (ASE) source filtered by two cascaded 50 GHz optical band-pass filters has been co-propagated together with a 40 G CP-QPSK signal over a 700 km fiber link, and the bit-error ratio of the data signal has been measured for different delta power values. As shown in FIG. 4, significant bit-error rate degradations of CP-QPSK signal over a large spectral region could be observed when increasing the power of the ASE source. This is due to the fact that the Cross Polarization Modulation (XPolM) induced random polarization rotations cover all of the above-mentioned categories: slow, fast and ultra fast. In this case, the fast polarization rotations were responsible for the observed impairment.
Cross polarization modulation effects are expected to affect not only standard coherent receivers based on digital signal processing, but also direct-detection receivers employing fast polarization controllers for input polarization demultiplexing. Indeed, current active polarization controllers can compensate only relatively slow polarization rotations (in the order of a hundred kHz) but would not be able to cope with fast polarization rotations induced by Cross Polarization Modulation (XPolM).
A large number of idler channels schemes, as well as transient suppression channels schemes based either on polarized/depolarized CW signals or on filtered ASE sources are known from the prior art. Cited, for example, is C. Headley, G. Agraval, “Raman Amplification in Fiber Optics Communication Systems” Academic Press, Dec. 30, 2004, or J. Chesnoy, G. Agrawal, I. P. Kaminow, and P. Kelley, “Undersea Fiber Communication Systems” Academic Press, October 3.
However, such conventional schemes have a severe impact on next generation transmission systems based on coherently detected polarization-multiplexed optical signals such as CPQPSK.
The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution that minimize the destructive Cross Polarization Modulation (XPolM) interactions in polarization-multiplexed transmission systems such as CP-QPSK.